This invention generally relates to a process for preparing .alpha.,.omega.-dicarboxylic acids.
More particularly, this invention provides a process for preparing saturated linear aliphatic .alpha.,.omega.-dicarboxylic acids containing 8 to 12 carbon atoms, by catalytic oxidation of the corresponding aldehyde acids or of the corresponding dialdehydes.
The industrial importance of saturated dicarboxylic acids is known. For example, to state only their main uses, dicarboxylic acids can be used as the primary components or as additives in the manufacture of various types of nylon, fibres and lubricants, and as additives for mineral oils etc.
When dicarboxylic acids are condensed with diamines, the resultant products can also be used as wetting agents and emulsifiers, in particular in the petroleum industry.
It is known to prepare saturated long-chain aliphatic .alpha.,.omega.-dicarboxylic acids by treating the ozonides of cycloolefines with oxidising agents. For example, in U.S. Pat. No. 3,328,250, cyclohexene ozonide is oxidised to adipic acid by ozone.
However, this process is difficult to carry out industrially, as it presents obvious dangers when working with large quantities of material, because it is necessary to evaporate the ozonide solution from the solvent before its oxidative decomposition to formic acid.
It is also known from U.S. Pat. No. 3,383,398 to prepare dicarboxylic acids in a single stage by ozonising cycloolefines in pyridine.
However, this process also finds no easy industrial application because of the high cost of the solvent (pyridine).
R. H. Perry J. and B. G. Coreman (A.C.S.--Div. Petrol Chem. Preprints 12 (2) D-5 to D-10 (1967)) indicate a method for preparing unsaturated long-chain aliphatic .alpha.,.omega.-dicarboxylic acids by the mono-ozonisation of polyunsaturated cycloolefines, followed by oxidation of the formed mono-ozonides in the presence of a heavy metal. This method is especially inconvenient industrially because of the difficulties, not easily overcome, which are encountered in separating the ozonide from the unreacted cycloolefines, in that the conversion of the polyunsaturated cycloolefine into mono-ozonide is very low, and thus at the end of the reaction there is a mixture the greater part of which is cycloolefine and the smaller part ozonide.
Methods are also known (GB No. 767,416, U.S. Pat. No. 3,804,895 and DOS No. 2,649,510) for preparing dicarboxylic acids by oxidising the corresponding aldehyde acids in the presence of oxidation catalysts in a quantity of at least 0.002% of the aldehyde acid by weight. However, with such processes the yields do not exceed 87.5% of the theoretical.